Boeing expects the grounding order on 737 MAX to be lifted in December and the plane to be back in commercial service in January as “one of the safest planes ever to fly,” the company announced Monday.

The plane was grounded after two fatal crashes that killed more than 300 people. Faulty sensors associated with the plane’s anti-crash software were blamed.

Boeing has been working with investigators of the Lion Air and Ethiopian Airlines crashes to correct the plane’s problems. The Federal Aviation Administration has been overseeing the plane’s rehabilitation process.

“We are taking the time to answer all of their questions,” Boeing said in a press release. “With rigorous scrutiny being applied, we are confident the MAX will be one of the safest airplanes ever to fly.”

The FAA last month demanded Boeing explain why it withheld documentation about employee concerns during the initial certification process. Instant messages obtained by CNN indicated pilots were concerned about the Maneuvering Characteristics Augmentation System.

Boeing CEO Dennis Muilenburg admitted in congressional testimony that the company had made mistakes with the plane.

Boeing said it is hopeful the order grounding the plane will be lifted and the updated training requirements to be instituted before the end of the year.

The return is dependent on five milestones, four of which are still pending. The one milestone completed last week was the multiday eCab simulator evaluation with the FAA “to ensure the overall software system performs its intended function,” both normally and during systems failures.

Still to be completed are simulator evaluations with pilots, certification flights with updated software, a final certification flight and Joint Operational Evaluation Board approval.

“We’re providing detailed documentation, had them fly in the simulators, and helped them understand our logic and the design for the new procedures, software and proposed training material to ensure that they are completely satisfied as to the airplane’s safety. The FAA and other regulatory authorities will ultimately determine return to service in each relevant jurisdiction. This may include a phased approach and timing may vary by jurisdiction,” Boeing said.

The 737 MAX has been grounded since the second crash, that of an Ethiopian Airlines flight in March that killed 157 people. The Lion Air flight went down in the Java Sea in Indonesia, killing the 189 passengers and crew aboard. The crashes were linked to a stabilization system that malfunctioned and kept pointing the planes’ noses down.

American and Southwest, the largest buyers of the 737 MAX, have taken the planes off their schedules into March.

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